It is 7:30pm on Friday and Christina is kneeling down to pray. To her right, a group of around 500 noisy protesters, armed with whistles, glow sticks, drums and “keep your rosaries off my ovaries” signs are interrupting with a chorus of “sit down if you hate women”.

Baker, six months pregnant and accompanied by her 13-year-old brother Lorenzo, is undeterred. She’s at the 40 Days for Life prayer vigil outside the British Pregnancy Advisory Service (BPAS), the UK’s largest abortion provider’s headquarters in Bedford Square, central London.

“If you are anti-abortion you should stand up for voiceless children,” she says. “I’m standing up for babies.”

“The trouble is the media is so pro-choice. At 12 weeks a baby is fully formed.”

As for the noisy counter-protest, it makes her feel "sick".

“It’s disgusting,” she says. “I don’t hate women… I am one at the end of the day."

Christina and around 300 others were there to see Alan Hopes, the Auxiliary Bishop of Westminster, who attended the prayer vigil.

Commenting afterwards, Hopes said he was there to stand up against the “culture of death”.

Kate Smurthwaite, vice chair of pro-choice group Abortion Rights, says she is concerned about a mainstream bishop supporting an "extremist" group.

"We think of 40 Days for Life as being an extremist OTT group," she says. "They would hold up graphic images and engage in tactics that I would never pursue whatever my opinions were.

"They think it’s acceptable to harass individual women, making very difficult situations immeasurably worse. The reason everyone was there is they were being joined by one of the leading Catholic bishops in London, someone who has regular meetings with politicians and is seen as a community leader.

"That’s terrifying to me. They've been filming women coming in and out of clinics, shouting at women. When I got here I was filled with incandescent rage that these people would stand outside a clinic and hold a religious service there.”

She and fellow pro-choice picketers banged drums, blew whistles, and chanted “shame on you” and “stop harassing women” to promote their cause.

Why? Because 40 Days has been accused of filming women entering abortion clinics, handing out incorrect information and harassing women during their prayer vigils outside BPAS.

Paul, a 29-year-old Catholic, says he was told by 40 Days to put his camera away at Friday’s protest because of accusations that the group had been filming women outside clinics. “I see women outside abortion clinics,” he says. “It’s a cliché but some are literally being dragged in by their partners. A lot of people feel they have no choice.”

But the allegations about 40 Days for Life were dismissed by Robert Colquhoun, one of the founding members of the organisation in Britain, who says he knew “God was calling me” to support the group. “We don’t encourage anyone to film," he says. “We’re here to pray for an end to abortion. Our campaign is focusing on prayer.”

So what does he think of the protesters? “The protesters range from rude to the aggressive behaviour we’ve seen on display tonight,” he replies.

Alfonso, another 40 Days for Life member, takes a different view. “The protest is good. Our campaign is working. We’re making them nervous.”

Those praying for an end to abortion at the event range from nuns to priests to young Catholics.

Nineteen-year-old Jo has done “sidewalk counselling” before, talking to women as they head into abortion clinics.

“Women don’t know what abortion is. I don’t think women are made aware,” says Jo. “Abortion is the termination of a human life.”

Jo is with her friend, 14-year-old Declan, whose views are clear. “We’re standing up for what’s right. Abortion is murder,” he tells me.

Then there’s Dr Christian Shell, proudly holding a sign picturing a foetus with the slogan ‘you take away my choice’ in capital letters; and Paulo Manca, who has brought eight of his 13 children along, including his four-year-old daughter. Is she pro-life? “She couldn’t have a conversation with you about it,” says Shell.

At the pro-choice protest, separated by a metal barrier, the attendees are just as diverse. London’s critical mass group rides past to show their support, with the Green Party’s London mayoral candidate Jenny Jones also in attendance. Jones says she’s “concerned” about any threat to the right to abortion.

Despite minor skirmishes, there’s no real trouble. The prayers end and the protests finish, but the policemen remain until the very end. “We don’t get any trouble from this lot,” says one officer. A pro-choice protester agrees, remarking as she leaves: “It’s just a stand-off really.”

So what's next? Smurthwaite says this is just the start.

“This is the beginning I think of US-style tactics coming to the UK,” she says.

“I really hope they’ll be less successful here. Last night there were lots of Catholics on our side. They don’t even represent the mainstream when it comes to their religion. Friday’s protest was the biggest pro-choice one in my activist life-span.”